Amplifier design is a study in compromise. Often a single device can't achieve optimal speed, drift, bias-current, noise, and output-power specs. Although various devices emphasizing one or more of these areas have evolved, those skilled in the art often find that the required performance can only be obtained with a dedicated amplifier design. Moreover, if a single device can't provide the desired characteristics (high speed and DC precision, for example), those skilled in the art can often configure a composite amplifier to do the job. Composite designs combine the best features of two or more amplifiers to achieve a level of performance unobtainable in a single device. Williams, "Composite Amplifiers Yield High Speed and Low Offset", EDN, Jan. 22, 1987.
One frequently occurring problem is that of DC coupling signals, such as in FM modulators and phase-locked loops. Heretofore, at least three techniques have been used. One technique is based on floating the power supplies of wideband, DC-coupled amplifiers so as to overcome their limited offset voltage range. Another technique is based on summing a high-speed, low level signal with a low-speed, high level signal by converting the former to a current, and dropping this current across a resistor, one end of which is connected to the high level signal. Still another technique is to conventionally sum the signals with a summing amplifier of combined high frequency capability.
Clearly a single amplifier design is preferred to a composite design. Not only are the number of parts reduced but often the size is reduced. Single component designs also offer the potential for reduced costs due to economies of scale. Since the problem of DC-coupling a high speed signal and a low speed signal has applicability to the transmission of video information, a single amplifier design would have great utility to the industry.